Longish bill, neck and head, central (in body) wings and full structure- all good for Common Tern
The dark trailing edge looks well defined but this could be due to photo effect- it should be rather diffuse in Common Tern (Arctic normally shows the well defined trailing edge). Translucency seems to be confined to the inner primaries which is a pro-Common Tern feature.
In this pose the bird looks short-billed with short neck and head and also looks like it has grey underparts with white cheeks. From this angle it also looks like the wings are off centre and appears long-tailed. All those features are pro-Arctic. It aint easy. Important to get a photo of the bird square on. The harsh light doesn't provide a neutral lighting to judge tone and a lot of a tones are probably shade.
This don't help either.
It's that tit-spanner time of year again- Common and Arctic Terns on the move. Every year we get single birds/small groups passing through the farmlands, often quickly and at distance and we usually deal with the identification with a combination of careful methodical analysis and judgement (mild stringing). Best way to attempt an identification I reckon is to try and get photos but birds can change apparent structure, size, tone, patterns and even colour depending on angle and light and also the camera often makes a degree of image capture interpretation and also doesn't always capture the fine detail you need unless the bird is close and photogenic. I reckon on balance this one is Common Tern.
A few other bits moving today too- about 20+ Swallow, a few Buzzard, a Red Kite, a couple of new Great Black-backed gull, a few Blackheads moving, a Reed Warbler has arrived and 'my' Whitethroat has arrived to the obs garden.